1. Field of the Invention
The present invention relates to an optical disc such as a CD, a DVD or a Blu-ray disc, an optical disc manufacturing method for manufacturing an optical disc, an optical disc recording device for recording information on an optical disc and an optical disc reproduction device for reproducing information from an optical disc.
2. Description of the Background Art
Conventionally, optical discs are widely used as inexpensive recording media for digital information. For example, a Blu-ray disc has a capacity of 25 gigabytes with a single layer and 50 gigabytes with two layers and can record high-vision and good-quality video contents about 2 to 4.5 hours. Accordingly, as the optical disc capacity increases, the value of digital contents recorded on one optical disc increases and the protection of the copyright of the recorded digital content has become an essential technical problem.
However, nowadays, there are a great number of illegal optical disc manufacturer such as pirate makers for illegally duplicating digital contents from optical discs. This hinders a sound distribution of digital copyrighted works and creates a situation where profits are not fairly distributed to copyright holders.
Accordingly, technology for changing the reflectivity of a reproduction film on concave and convex marks by laser irradiation used for tracking of a guide groove in an optical disc having a digital copyrighted work recorded by the concave and convex marks is disclosed, for example, in patent document 1.
By using the optical disc disclosed in patent document 1, optical discs which need not be collected can be provided even if they are used, for example, for rental.
Further, for example, patent document 2 discloses technology for recording sub-information by locally changing the reflectivity of an information recording surface at a position at a specified distance from the edge of a mark or space based on a sub-data string for marks or spaces with a specified length or longer out of concave and convex marks.
According to the invention disclosed in patent document 2, the reflectivity of the mark or space is locally changed at such a timing as not to influence the positional information of the edge of the concave and convex mark. This enables the recording of sub-information that makes illegal copying difficult without influencing reproduction of a main data string represented by a mark string by an optical pickup.
Further, for example, patent document 3 discloses an information recording/reproducing method by which, using a recording medium which is optically changed depending on the quantity of irradiated laser light, laser light intensity modulated by a signal having the same band as a first signal obtained by modulating a signal such as a video or a data signal according to the quantity of laser light and having a recording state ON/OFF controlled by a second signal in a band lower than that of the first signal is irradiated twice or more to the same information track on the recording medium having the first signal already recorded with positions on the recording medium synchronized, the second signal is recorded in a superimposition manner to further optically change a part irradiated with the laser light, and the second signal is separated and reproduced at the time of reproduction.
According to the invention disclosed in patent document 3, by recording the second signal on the same information track in a superimposition manner as information indicating the deletion of the information of the first signal or information indicating the position of an information track as an alternative to the information of the first signal, the management of the information track on the recording medium can be realized without judging whether the information of the information track is valid or invalid or providing a special information track for managing the position of the information track as an alternative to the information track.
Further, for example, patent document 4 discloses an optical disc including a spiral first information track having specified data recorded thereon and a spiral second information track formed in a region between track parts of the first information track and having copy detection information recorded thereon.
According to the invention disclosed in patent document 4, by recording detection information used for copy disc discrimination on the second information track different form the first information track for data, an illegally copied disc can be effectively checked without influencing a normal operation. Further, by recording security information on the second information track, the illegal duplication of the security information can be prevented.
However, by any of these methods, it is difficult to record medium unique information of the optical disc while preventing the illegal duplication without increasing cost required for the manufacturing of the optical disc and without deteriorating the reading accuracy of main information. It is also difficult to record the medium unique information of the optical disc while efficiently preventing illegal duplication without sacrificing the recording region for the main information.
These are for the following reasons. In the invention disclosed in patent document 1, the guide groove is formed on the optical disc beforehand in order to change the reflectivity of the irradiated part by irradiating a recording laser beam of a specified intensity or higher to the optical disc. Normally, in the case of manufacturing an optical disc formed with a guide groove and concave and convex marks, it is necessary to record the guide groove to locate the concave and convex marks in the center after the concave and convex marks are recorded in an optical disc master or to record the concave and convex marks after the guide groove is recorded. However, in the case of a Blu-ray disc, accuracy in the order of several tens nanometers in a radial direction is necessary for positioning between these tracks, and it is difficult to record both the guide groove and the concave and convex marks using a normal mastering apparatus. In order to realize this, a special mastering apparatus is necessary and an increase of cost required for the manufacturing of an optical disc is unavoidable.
In the invention disclosed in patent document 2, laser light is irradiated to a position at the specified distance from the edge of the mark or space with the specified length or longer. Thus, it is necessary to search and save the mark or space with the specified length or longer, whereby a recording time for recording the sub-information is uselessly consumed and cost required for the manufacturing of the optical disc increases. In the case of recording the sub-information, for example, only on a synchronization code known to have a mark or space with the specified length or longer, a corresponding recording region is necessary to record the medium unique information of a hundred and several tens bytes. Therefore, in this case as well, the recording time or reproduction time is uselessly consumed.
In the invention disclosed in patent document 3, the second signal is recorded on the part prerecorded with the first signal a plurality of times in a superimposition manner, and this recording is repeated until the second signal can be normally read. Accordingly, the reading accuracy of the prerecorded first signal deteriorates and a recoverable defect margin such as an error correction deteriorates.
In the invention disclosed in patent document 4, the second information track having the security information recorded thereon is formed in the partial region between the track parts of the first information track and the first and second information tracks are arranged such that the position of the second information track is reached upon a movement from the first information track by one track pitch. However, the formation of the second information track between the track parts of the first information track means to double the track pitch of the adjacent parts of the first information track, wherefore the region for recording the main information is sacrificed by forming the second information track. In other words, recording capacity per optical disc is reduced by forming the second information track.    Patent Document 1:    Japanese Unexamined Patent Publication No. H09-306030    Patent Document 2:    Japanese Patent No. 3454410    Patent Document 3:    Japanese Patent No. 2903422    Patent Document 4:    Japanese Unexamined Patent Publication No. H08-147767